1. Field of the Invention
The present invention relates generally to a system and method to obtain density changes in fluid flow around crafts and objects and more particularly to using Schlieren flow visualization.
2. Description of the Related Art
It is important to be able to characterize fluid flow, such as air current, around aircraft and other objects (such as windmills) in order to maximize efficiency of these systems. One method of doing so is to use Schlieren flow visualization. Schlieren flow visualization is based on the deflection of light by a refractive index gradient. The index gradient is directly related to flow density gradient. The deflected light is compared to undeflected light at a viewing screen. The undisturbed light is partially blocked by a knife edge. The light that is deflected toward or away from the knife edge produces a shadow pattern depending upon whether it was previously blocked or unblocked. This shadow pattern is a light-intensity representation of the expansions (low density regions) and compressions (high density regions) which characterize the flow.
Currently, this method has been used in wind tunnels to characterize fluid flow around aircraft models and engine components. This is done via the flow visualization technique described above. More specifically, the method uses Schlieren photography to obtain fluid flow data. Schlieren photography is similar to the shadowgraph technique and relies on the fact that light rays are bent whenever they encounter changes in density of a fluid. Schlieren systems are used to visualize the flow away from the surface of an object. One example of a Schlieren system uses two concave mirrors on either side of the test section of the wind tunnel. A mercury vapor lamp or a spark gap system is used as a bright source of light. The light is passed through a slit which is placed such that the reflected light from the mirror forms parallel rays that pass through the test section. On the other side of the tunnel, the parallel rays are collected by another mirror and focused to a point at the knife edge. The rays continue on to a recording device like a video camera.
If the parallel rays of light encounter a density gradient in the test section, the light is bent, or refracted. If a shock wave has been generated by a model placed in the supersonic flow of the wind tunnel, the ray of light passing through the shock wave is bent. This ray of light does not pass through the focal point, but is stopped by the knife edge. The resulting image recorded by the camera has darkened lines that occur where the density gradients are present. The model completely blocks the passing of the light rays, so a black image of the model is seen. But more importantly, the shock waves generated by the model are now seen as darkened lines on the image. Hence, the method allows one to visualize shock waves.
However, when using the method in a wind tunnel, a user will not be getting fluid flow visualization of an aircraft or other object's actual performance in the field, but, rather, a simulation. In addition, wind tunnel testing is very expensive.
It has also been contemplated to simply use the edge of the sun as a background for a Schlieren fluid flow analysis of aircraft fluid flow. However, simply using the edge of the sun provides an extremely rough estimate of the fluid flow.
Therefore, it is desired to provide a cost effective system and method to obtain relatively detailed fluid flow characterization around aircraft and other objects during actual operation.